The US president and UK Prime Minister David Cameron have both said a significant drawdown of troops would begin in 11 months' time. But Gen Petraeus appeared unconvinced, saying he would "certainly" advise Mr Obama if he thought the goal was unrealistic.

In his first major interview since assuming command of more than 140,000 coalition troops in Afghanistan last month, he said the reality "on the ground" would dictate whether it was possible.

In what may be interpreted as a challenge to Mr Obama, Gen Petraeus praised former president George W Bush for making the "tough call" and "very courageous decision" in ordering the Iraq troop surge and sticking with it despite "a number of difficult months".

Gen Petraeus, who took command of the Afghan mission last month, was speaking in an interview with American television network NBC's 'Meet the Press'. Support for the war and Mr Obama's handling of it are at an all-time low, while the monthly death toll for American troops hit a record high of 66 in July.

The general was speaking in his first major media outing since taking over from General Stanley McChrystal, who was sacked after criticising senior members of the Obama administration in a 'Rolling Stone' magazine interview.

While the White House has consistently emphasised the importance of the July 2011 date -- when Mr Obama's 2012 re-election campaign will begin in earnest -- the Pentagon has repeatedly downplayed its significance.

Advice

Gen Petraeus said his advice would be based on military -- not political -- considerations.

"Certainly, I am aware of the context within which I offer that advice," he said. "But that just informs the advice, it doesn't drive it. The situation on the ground drives it."

The anti-war Left fears that Gen Petraeus, who is revered by Americans for his central role in turning around the situation in Iraq, will be in a strong enough position to be able to prevail over the war sceptics in the White House, including, perhaps, Mr Obama himself.